Field of Invention
The invention relates to a power converting technique and more particularly to a resonant converter and a controlling method thereof.
Description of Related Art
The development trend of direct current (DC) converters, similar to that of most power supply products, is toward high efficiency, high power density, high reliability, and low costs. Since resonant converters (e.g., LLC resonant converters, etc.) are capable of performing zero-voltage switching (ZVS) in the primary winding and zero-current switching (ZCS) in the secondary winding on the full-load range, the resonant converters have gradually been applied as DC converters in recent years.
As to the circuit design of resonant converters, the over-current protection is a crucial issue. In general, the resonant circuit may produce significant resonant current on the condition of over-load or short-circuit. If the resonant current is not limited or protected, the resonant converter is very much likely to be damaged by high current.
At present, one of the feasible solutions to realize the over-current protection is to add a clamp circuit to the resonant converter. By clamping the voltage across the resonant capacitor at the level of the input voltage, the over-current protection may be realized. The foregoing method is easy to execute, and periodic current limitation may be achieved without configuring any additional control circuit, which is powerless control. To be specific, in the resonant converter which adopts the aforesaid over-current protection, several clamping diodes are required to be added to the circuit, and the resonant circuit is designed to have the symmetrical structure.
However, in the resonant converter adopting the over-current protection mechanism, the voltage across the resonant capacitor is clamped according to the input voltage, so that the cross voltage of the resonant capacitor may change following the change to the input voltage, and the maximum voltage across the resonant capacitor can merely be the input voltage. Hence, the design of the resonant circuit has some limits, and the operation range of the resonant circuit may be affected.
In addition, when the resonant converter goes into the hold up time, since the cross voltage of the resonant capacitor is limited by the clamping diode, the stored energy of the resonant circuit may be decreased and thus decreasing the maximum gain of the output voltage. As a result, designers may have to use large capacitors to comply with the requirement of the hold up time, and problems regarding the increase in size and costs of the resonant converter may occur.